The accident took place at Lamaha forest in Rautahat district when the bus with a Nepali number plate collided with the Indian bus along the East-West Highway, a police official said.
While the Nepalese bus was heading towards east, the Indian bus was heading towards Pokhara, a major tourist spot.
Police said all the 15 injured were travelling in the Nepalese bus. They are undergoing treatment at a local health centre in Rautahat.
Drivers of both the buses have been taken into custody for investigation, according to the police.